1. Field of the Invention
The present invention generally relates to an apparatus for measuring the characteristics of an optical fiber. More specifically, the present invention relates to an apparatus for measuring the characteristics of an optical fiber, wherein the apparatus is configured to launch an optical pulse generated from a coherent light into an optical fiber, mixing the coherent light and a Brillouin backscattered light from the optical fiber to obtain a mixed light signal, converting the mixed light signal into an electric signal, and finding the characteristics of an optical fiber based on the electric signal.
Priority is claimed on Japanese Patent Application No. 2006-336200, filed Dec. 13, 2006, the content of which is incorporated herein by reference.
2. Description of the Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the present application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
There has been established a first conventional method of measuring the strain distribution and the temperature distribution along an optical fiber in an environment in which the optical fiber is installed. The first conventional method measures the center frequency of a Brillouin scattered light that is generated from the optical fiber into which an optical pulse has been launched. This first conventional method uses the installed optical fiber as a medium for detecting the strain and the temperature thereof. This first conventional method simplifies the configuration that needs to measure the strain distribution and the temperature distribution as compared to when using a lot of point sensors.
Typical examples of the first conventional method may include Brillouin optical time domain reflectometry (BOTDR) and Brillouin optical time domain analysis (BOTDA).
The Brillouin optical time domain reflectometry (BOTDR) is a method to measure the Brillouin frequency shift of a spontaneous Brillouin backscattered light, which has been reflected by acoustic wave that varies in velocity depending upon the strain and the temperature. Namely, the Brillouin optical time domain reflectometry (BOTDR) is a method of detecting the Brillouin backscattered light that is emitted from an input end of an optical fiber after a pulse light has been launched into the same end of the optical fiber. The Brillouin optical time domain reflectometry (BOTDR) is disclosed in Japanese patents Nos. 2575794 and 3481494.
Brillouin optical time domain analysis (BOTDA) is a method of measuring a varying component of a probe light, wherein the variation of the probe light appears due to Brillouin stimulated-scattering phenomenon that is caused by a pumping light or pulse light having a higher intensity than a threshold when the pumping light or pulse light is launched into one end of an optical fiber, while the probe light is launched into the other end of the optical fiber. This Brillouin optical time domain analysis (BOTDA) is disclosed in Japanese patent No. 2589345.
It has been known for the Brillouin optical time domain reflectometry (BOTDR) and the Brillouin optical time domain analysis (BOTDA) that narrowing the pulse width that is launched to the optical fiber improves the spatial resolution. Extensively narrowing the pulse width beyond the narrower limit makes it difficult to accurately measure the center frequency of the Brillouin scattered light. The spatial resolution is ranged about 2 m or 3 m.
A distributed optical fiber sensing system utilizing transient phenomenon of an acoustic wave has been proposed in order to improve the spatial resolution of the measurement method that uses the stimulated Brillouin scattered light. This system is disclosed in Report, Vol. 105, No. 242, OFT2005-16, P.1-6 (2005). This distributed optical fiber sensing system is realized by taking into account the transient phenomenon that inhibit instantaneous start up oscillation since the acoustic wave causing the Brillouin scattering is the mechanical oscillation. After a first pump light is propagated through an optical fiber, then a second pump light that causes Brillouin scattered light for measurement is propagated through the optical fiber, thereby preventing the transient phenomenon from appearing on the Brillouin scattered light for measurement, and realizing a high spatial resolution of about 10 cm.
Another device has been proposed for realizing a high spatial resolution of centimeter-order, which is so called Brillouin optical correlation domain analysis (BOCDA) that is different from a time domain measurement. This proposal is disclosed in Japanese patent No. 3667132. This device uses a frequency converter to change the center frequency of a probe light so that the difference in center frequency between a pump light and the probe light approaches among the Brillouin frequency shift. Then, the device performs frequency modulation to a light source, thereby causing a power transfer from the pump light to the probe light at a position where the pump light and the probe light are synchronized in phase. The device uses a photo detector that detects the optical power of the probe light that has been emitted from the optical fiber, thereby measuring Brillouin spectrum at the position where the pump light and the probe light are synchronized in phase. This device realizes a high spatial resolution of about 1 cm. This method is so called Brillouin optical correlation domain analysis (BOCDA).
OFT2005-16, P.1-6 and Japanese patent No. 3667132 need that measuring lights are launched into both ends of an optical fiber. These documents fail to propose improvement of the spatial resolution of the Brillouin optical time domain reflectometry (BOTDR).
The Brillouin optical time domain analysis (BOTDA) and the Brillouin optical correlation domain analysis (BOCDA) need that the pump light and the probe light are launched into both ends of an optical fiber. The device realizing the Brillouin optical time domain analysis (BOTDA) or the Brillouin optical correlation domain analysis (BOCDA) is likely to be complicated in structure and highly expensive.
It would have been non-common sense and non-obviousness to try to a person having ordinary skilled in the art to realize an apparatus and a method of realizing a high spatial resolution of the Brillouin optical time domain reflectometry (BOTDR).
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved device and method of realizing a high spatial resolution of the Brillouin optical time domain reflectometry (BOTDR). This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.